1. Field of the Invention
This invention relates to gas sorption compositions, apparatus, and associated processes, for effecting the sorptive removal of hazardous gases such as are widely encountered in the manufacture of semiconductor devices, such as silane, dichlorosilane, trichlorosilane, silicon tetrachloride in conjunction with sorptive removal of ammonia. The present invention further relates to a process of removing such compounds from gas streams with a single adsorbent.
2. Description of the Related Art
Silicon nitride films are extensively used in both silicon and gallium arsenide device technology. Silicon nitride is a dense insulator with a dielectric constant of 5.8-6.1, a refractive index of 1.98-2.05, and a density between 2.3 and 2.8 g/cm.sup.3. Unlike silica or phosphosilicate glass, it is an excellent barrier to alkali ion migration and is virtually impermeable to water, so it is used extensively as a cover layer in MOS technology. Its use has allowed unencapsulated circuits to be practical in many consumer applications. Because it restricts the diffusion of gallium, it is used as a "capping" material for gallium arsenide during the high temperature anneals that must be carried out after ion implantation steps. It is about 100 times more resistant to thermal oxidation than silicon, so that it can be used as a mask in a number of silicon-based VLSI processes which require selective oxidation of the semiconductor. It is used in radiation-hardened devices because of its superior radiation resistance over silicon dioxide.
A layer of silicon nitride may be grown on silicon by direct nitridation, but this process must be carried out at high temperatures (1000.degree.-1300.degree. C.) and is extremely sensitive to even trace amounts of water or oxygen. Furthermore, film thickness is greatly restricted by the low diffusivity of nitrogen through silicon nitride. Therefore, all silicon nitride films used in silicon and gallium arsenide technology are deposited, most commonly by chemical vapor deposition (CVD).
Silicon nitride films may be formed by chemical vapor deposition using silane and ammonia, hence: EQU 3SiH.sub.4 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +12H.sub.2
Deposition can be accomplished at atmospheric as well as at reduced pressures. Typically large excesses of the nitrogen source ammonia are used in order to obtain optimal silicon nitride stoichiometry.
Silane is extremely flammable, reacting with air explosively in even low dilutions. Thus, some nitride systems use chlorosilanes which are safer to handle, as for example: EQU 3SiH.sub.2 Cl.sub.2 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +6HCl
An excess of ammonia is again generally used, but the ratio of NH.sub.3 to chlorosilane does not need to be as high as when silane is used as the silicon-containing gas component.
The effluent gas stream from silicon nitride CVD reactors typically contains large amounts of unreacted silane or chlorosilane and ammonia, as well as reaction by-products such as hydrogen chloride and ammonium chloride in the chlorosilane process. These components must be scrubbed from the effluent gas stream, to very low levels, so that they are not released to the environment or to downstream equipment such as vacuum pumps.
The "Threshold Limit Value" or TLV (American Conference of Governmental Industrial Hygienists: "Threshold Limit Values and Biological Exposure Indices," 1987, Cincinnati, Ohio) and analogous "Permissible Exposure Limit" or PEL (U.S. Department of Labor, Occupational Safety and Health Administration: 29 CFR 1910.1000) are concentrations of gases in workplace air, below which it is believed that routine exposure poses no risk of harm to personnel. These concentrations are thus benchmarks for effluent gas scrubbing. An adequate scrubber must remove hazardous gas components from the effluent gas stream to concentrations below the TLV or PEL.
Silane is considered toxic; its TLV and PEL are set at 5 parts per million (ppm). The consequences of release of silane into the workplace can be dire, for, in addition, it is extremely flammable, reacting with oxygen explosively even in very dilute form. The lower flammable limit for silane in air is 1.5%. For safety considerations, chlorosilanes are often used as alternative silicon sources. These materials are toxic and corrosive. They decompose readily in moist air or on mucous membranes to form oxidized silicon species and hydrogen chloride. Because of their ready hydrolysis to form HCl, TLVs and PELs have not been formulated for all chlorosilane compounds, but the TLV of HCl is used (5 ppm).
Ammonia is toxic and corrosive, as is hydrogen chloride. TLV and PEL for ammonia are 25 ppm and 50 ppm respectively, and for hydrogen chloride both TLV and PEL are set at 5 ppm.
When the silicon nitride is formed by the chlorosilane plus ammonia CVD process, large amounts of by-product ammonium chloride may be formed. This ammonium chloride is abrasive, and if it is not scrubbed from the effluent stream or prevented from forming, it is very destructive to vacuum pumps and any other downstream equipment. By-product ammonium chloride can also clog downstream ducts resulting in onerous maintenance and down-time for the facility.
A good effluent gas scrubber must not only remove hazardous gas components to below TLV/PEL, but must also possess several other attributes. It must operate safely, with no risk of explosion or spillage. It should have high capacity for the hazardous components, so that it need not require an extremely large volume for scrubbing or frequent change-outs. It must have high kinetic efficiency for scrubbing, so that high flow rate effluent gas streams may be scrubbed. Scrubbers of a simple, passive design are preferred, since they are likely to be more economical. Finally, the scrubber should convert the hazardous components of the effluent gas stream to stable, environmentally acceptable species that may be disposed of safely and economically.
The capacity of the scrubber is considered to be that amount of the hazardous gas component that can be sorbed per unit of scavenger, before breakthrough occurs. Breakthrough occurs when the hazardous component passes through the scrubber in concentrations higher than its TLV or PEL. Breakthrough may be monitored by a variety of commercially available hazardous gas monitors such as the MDA Toxic Gas Detector or the Gas Tech Detector Tubes, or colorimetrically.
In general, the known methods of scrubbing the effluent gas streams from CVD reactors include wet scrubbing, combustion (so-called "burn boxes"), plasma scrubbing, and dry scrubbing methods such as activated carbon.
Using wet scrubbing, the exhaust gas stream may be processed using aqueous solutions of relatively cheap reagents such as sodium hydroxide or potassium permanganate. However, wet scrubbing requires a large gas treating unit and the resulting large volumes of aqueous waste solutions may present problems as to environmentally acceptable disposal. In addition, the aqueous solutions used are very corrosive, and they thus can corrode fittings and connections and present risks to personnel and equipment in the event of an equipment failure. Commercially available aqueous ammonia scrubbers can scrub silane or chlorosilanes, but in the process they generate hydrous silica which clogs filters. In addition, the efficiency of aqueous scrubbers for silane is usually very low.
In combustion processes, the waste gases are brought into contact with air in a combustion chamber and are burned. When silane is used, such processes may present an unacceptable explosion hazard. In addition, combustion is not applicable to highly corrosive effluent gas streams such as are encountered from silicon nitride CVD processes.
The exhaust gas from the CVD reactor may be scrubbed by passing it through a plasma chamber (see Hammond, M. L., "CVD Exhaust-Safety and Environmental Sanity," Proc. Eur. Conf. Chem. Vap. Deposition, 8th, C2/449-C2/457, 1991). However, such a method requires fairly complicated, expensive equipment and is limited in application to low pressure processes.
The use of activated carbon beds to physically adsorb hazardous constitituents from semiconductor manufacturing gas streams is well established (see Calgon Carbon Corporation, "Ventsorb.RTM. for Industrial Air Purification," Bulletin 23-56b, 1986). Activated carbon, while highly efficient, scavenges hazardous gas constituents by both physical and chemical adsorption. Physically adsorbed constituents can later desorb unless the carbon is periodically treated by carefully controlled oxidation. The use of activated carbon to treat silane is especially undesirable because of the extreme flammability of silane and the combustible nature of carbon.
Dry scrubbing methods for silane have been described, wherein the silane reacts with a scavenger such as a metal hydroxide or metal salt immobilized on a solid support material (Kitayama, M., et al., U.S. Pat. No. 4,535,072; Gokcek, C., U.S. Pat. No. 5,024,823). In addition, similar dry scrubbing methods for removing ammonia from air or gas streams comprising hydrocarbons have been described (Chao, C., and Rastelli, H., U.S. Pat. No. 5,019,667). Such dry scrubbing methods have the advantage of being passive, simple systems. However, the simultaneous scrubbing of highly basic ammonia and the hydride gas silane with a single variety of scrubber or scavenger material has not been addressed.
Therefore, it would be a substantial advance in the art to provide gas sorption compositions which may be usefully employed to scrub the effluent gas stream from silicon nitride CVD processes and to remove all of the hazardous gas components, both from atmospheric pressure processes as well as in low pressure processes.
Accordingly, it is an object of the present invention to provide a method for cleaning an exhaust gas containing ammonia and a silicon-containing gas component such as silane and/or a chlorosilane compound.
Another object of this invention is to provide a dry scrubbing method for removing ammonia and a silicon-containing gas component(s) from an exhaust gas, providing a waste material that is very compact and which is improved in safety and environmental acceptability.
A still further object of this invention is to provide a method for cleaning an exhaust gas using a scavenger that has a high kinetic efficiency and a high capacity for ammonia and a silicon-containing gas component such as silane and/or a chlorosilane compound.